1. Field of the Invention
This invention relates to storage control technology.
2. Description of the Related Art
In a database system which handles vast amounts of data, such as for example a data center, a storage system configured separately from a host computer is used to manage data. Such a storage system is for example a disk array system such as a RAID (Redundant Array of Independent Inexpensive Disks) system, in which numerous media drives are arranged in an array. As the media drives provided in the storage system, for example, hard disk drives having a SATA (Serial ATA)-specification interface (hereafter, “SATA-HDD”) may be adopted (see for example Japanese Patent Laid-open No. 2004-348876).
A HDD generally comprises the hard disks, and a control portion (for example, a control board and recording heads) which control data accessing of the hard disks. The control portion receives a read command specifying a data block group, and for each data block in the data block group, read processing to read the data block is performed, so that such data block groups can be read. If a certain data block cannot be read even when read processing is performed, the control portion attempts to read the data block which could not be read in one read processing trial by performing repeated read processing (that is, a read retry) to read the same data block.
Depending on the HDD state, there may occur cases in which read retries occur frequently in order to read a plurality of data blocks specified by a read command, so that a substantial amount of time may be required to read the plurality of data blocks.
One method of preventing this read time from becoming too long is a method of limiting the read time length. A HDD generally operates based on a variety of parameters set for the HDD. Depending on the type of HDD, these parameters may include a parameter related to limitation of the read time length (hereafter, a “limit parameter”), and in some cases it is possible to change the value of this limit parameter. Such a limit parameter may be, for example, the retry time, and/or the number of retries. The retry time is the amount of time required for a read retry of one data block, or, the total amount of time for one or more read retry trials performed when reading a data block group. The number of retries may be, for example, the maximum number of read retry trials for the same data block, or, the maximum number of read retry trials which can be performed when reading a data block group.
One type of HDD which enables modification of limit parameter values is, for example, an HDD having an interface which conforms to the fibre channel standard (hereafter an “FC-HDD”). The control portion of an FC-HDD can return an error (for example, time-out) if it is not possible to read a plurality of data blocks specified by a read command according to the value of a limit parameter set for the FC-HDD.
Such a circumstance may only occur when reading from a certain area on a hard disk, or may occur when reading from numerous areas. In the latter case, it is necessary to block the HDD. In the former case, if the HDD is similar to an FC-HDD, it should be possible to handle the problem by modifying the value of a limit parameter (for example, by increasing the value of the number of retries).
However, even in the former case, the above measure is not possible for HDD types which do not allow modification of limit parameter values. Hence there is the possibility that HDDs are blocked more than is necessary. One such HDD type is for example an SATA-HDD.
Hence an object of this invention is to prevent blockage, to an extent beyond what is necessary, of media drives having unmodifiable values of parameters relating to limits on read time lengths.
Other objects of the invention will become clear from the following explanation.